This invention relates generally to a method of making thermosetting composites and the resulting products, which preferably comprise electrical circuit laminate materials and coatings for heating elements. More particularly, this invention relates to an electrical circuit laminate or a coating for a heating element comprising a thermosetting resin of polybutadiene or polyisoprene that is subjected to a high temperature cure at a temperature of greater than about 250xc2x0 C., and which may optionally include (1) a polymer that crosslinks with the polybutadiene or polyisoprene resin; (2) a woven fibrous web impregnated with the resin; (3) discontinuous fibers dispersed within the resin system; and (4) optional inorganic particulate filler such as silica, titania and the like.
Commonly assigned U.S. Pat. No. 5,223,568 (which is fully incorporated herein by reference) describes a thermosetting composition which is particularly useful for making electrical substrate materials. In general, U.S. Pat. No. 5,223,568 describes a composition formed from the steps of:
(a) providing a moldable thermosetting composition that includes 1) polybutadiene or polyisoprene resin that is a liquid at room temperature and that has a molecular weight less than 5,000 and 2) a solid butadiene or isoprene-containing polymer capable of cross-linking with the polybutadiene or polyisoprene resin;
(b) forming the composition into a shape; and
(c) curing the composition to produce the electrical substrate material, including subjecting the composition to a high temperature cure condition at a temperature greater than about 250xc2x0 C. and less than the decomposition temperature of the composition. This composition thus comprises a two component system: the first component is the polybutadiene or polyisoprene resin, and the second component is the solid butadiene or isoprene-containing polymer. All of the components are subjected to the high temperature curing cycle (e.g., greater than 250xc2x0 C.). In preferred embodiments, the solid polymer is a thermoplastic elastomer block copolymer.
U.S. Pat. No. 5,223,568 also describes a composition with a dielectric filler (i.e., a material having a dielectric constant greater than about 1.2 at microwave frequencies) homogeneously dispersed throughout the composition to the extent that, when the composition is cured, the properties of the cured article (e.g., dielectric constant and coefficient of thermal expansion) do not vary more than about 5% throughout the article.
In preferred embodiments, the composition of U.S. Pat. No. 5,223,568 further includes a crosslinking agent capable of co-curing (i.e., forming covalent bonds) with the polybutadiene or polyisoprene resin, the solid copolymer, or both. Examples of preferred crosslinking agents include triallylcyanurate, diallylphthalate, divinyl benzene, a multifunctional acrylate, or combinations of these agents. When the electrical substrate material includes a dielectric filler, the volume % of the filler (based upon the combined volume of resin, thermoplastic elastomer, crosslinking agent (if any) and filler) is between 5% and 80%, inclusive. Examples of preferred fillers include titanium dioxide (rutile and anatase), barium titanate, strontium titanate, silica (particles and hollow spheres), corundum, wollastonite, polytetrafluoroethylene, aramide fibers (e.g., Kevlar), fiberglass, Ba2Ti9O20, glass spheres, quartz, boron nitride, aluminum nitride, silicon carbide, beryllia, and magnesia. They may be used alone or in combination.
The method disclosed in U.S. Pat. No. 5,223,568 provides a wide variety of shaped articles having favorable isotropic thermal and dielectric properties. These properties can be tailored to match or complement those of ceramic materials, including gallium arsenide, alumina, and silica. Thus, the cured articles can replace ceramic materials in many electronic and microwave applications: e.g., as specialized substrates for high speed digital and microwave circuits. Examples of microwave circuits include microstrip circuits, microstrip antennas, and stripline circuits. The cured products are also useful as rod antennas and chip carriers.
Polymeric coatings have also been used to cover electrical resistance heating elements. Electrical resistance heating elements used in connection with water heaters have traditionally been made of metal and ceramic components. A typical construction includes a pair of terminal pins brazed to the ends of an Nixe2x80x94Cr coil, which is then disposed axially through a U-shaped tubular metal sheath. The resistance coil is insulated from the metal sheath by a powdered ceramic material, which is usually magnesium oxide.
While such conventional heating elements have been the standard for the water heater industry for decades, there have been a number of widely recognized deficiencies. For example, galvanic currents occurring between the metal sheath and any exposed metal surfaces in the tank can create corrosion of the various anodic metal components of the system. The metal sheath of the heating element, which is typically copper or copper alloy, also attracts lime deposits from the water, which can lead to premature failure of the heating element. Additionally, the use of brass fittings and copper tubing has become increasingly more expensive as the price of copper has increased over the years.
As an alternative to metal elements, at least one plastic sheath electric heating element has been proposed in U.S. Pat. No. 3,943,328 to Cunningham. In the disclosed device, conventional resistance wire and powdered magnesium oxide are used in conjunction with a plastic sheath. Since this plastic sheath is non-conductive, there is no galvanic cell created with the other metal parts of the heating unit in contact with the water in the tank, and there is also no lime buildup. However, these plastic-sheath heating elements are not capable of attaining high wattage ratings over a normal useful service life, and thus have not been widely accepted. As an alternative, U.S. Pat. Nos. 5,586,214 and 5,930,459 to Eckman et al., which are fully incorporated by reference herein, disclose a heating element having an element body, a supporting surface on the element body, and a resistance wire wound onto the supporting surface that is coated with thermally conductive polymeric coating. This polymeric coating hermetically encapsulates and electrically insulates the resistance wire. While well-suited for its intended purposes, there nonetheless remains a need for alternative coatings that provide the desired electrical and protective properties required for electrical resistance heating elements.
The above-discussed and other problems and deficiencies of the prior art are overcome or alleviated by the electrical substrate material of the present invention. In accordance with the present invention, it has been discovered that:
(1) in a two component system, the first component (e.g., polybutadiene or polyisoprene resin) is not required to (a) be liquid at room temperature, (b) have a molecular weight less than 5,000, or (c) have pendent vinyl groups, nor is the second component (e.g., the unsaturated butadienexe2x80x94or isoprene-containing polymer) required to be a xe2x80x9csolidxe2x80x9d polymer;
(2) the thermosetting composition need not be a two component system, but instead may comprise a single polybutadiene or polyisoprene containing resin component (e.g., without the butadiene or isoprene-containing polymer), or the single resin component may comprise a polybutadiene or polyisoprene-containing polymer such as SBS triblock polymers; and
(3) unwoven fibrous webs may be incorporated into dielectric resins of the type described in U.S. Pat. No. 5,223,568 and in the resins systems of (1) and (2) above, whereupon the deficiencies in the prior art related to undesirable dimensional stability and brittleness are dramatically improved with only a relatively small loss in electrical performance. In addition, the resulting electrical laminate may be produced at a relatively low cost, thus permitting the use of the present invention in high volume applications, particularly high volume microwave applications and those uses requiring thin, low cost laminates.
The foregoing discovery is premised, at least in part, on the understanding that a critical part of the invention in U.S. Pat. No. 5,223,568 is heating the thermosetting material to a temperature greater than about 250 C. This high temperature cure step is unconventional for diene polymers such as polybutadiene or polyisoprene resins, with or without the presence of usual or other curatives such as peroxide or sulfur. Thus, it is the high temperature curing step of a polybutadiene or polyisoprene resin that defines this invention. The use of (a) a two component system, (b) a liquid resin, (c) a resin having a MW less than 5,000, (d) pendent vinyl groups, or (e) a solid butadiene or isoprene polymer are not required or even desirable under certain circumstances. The process of this invention thus comprises:
Two Component System
A forming process for producing an article such as an electrical circuit substrate comprising the steps of:
(a) providing a thermosetting composition comprising a polybutadiene or polyisoprene resin; and an unsaturated polybutadiene or polyisoprene containing polymer capable of participating in cross-linking with the polybutadiene or polyisoprene resin during cure; and
(b) curing the composition to produce the article (e.g., electrical circuit substrate) by subjecting the composition to a high temperature cure condition at a temperature greater than about 250 C. and less than the decomposition temperature of the composition.
One Component System
A forming process for producing an article such as an electrical circuit substrate comprising the steps of:
(a) providing a thermosetting composition comprising a polybutadiene or polyisoprene resin; and
(b) curing the composition to produce the article (e.g., electrical circuit substrate) by subjecting the composition to a high temperature cure condition at a temperature greater than abut 250 C. and less than the decomposition temperature of the composition.
The electrical substrate material of this invention may also include a plurality of woven webs (such as E-glass webs) embedded in a mixture of the polybutadiene or polyisoprene based resin system and optional inorganic filler (e.g., silica) laminated between one or two sheets of conductive foils (e.g., copper) to produce a circuit board material that is especially well suited for microwave applications. Of course, if very thin (e.g., less than 5 mil thickness) cross-sections are desired, then only a single saturated web may be used for the dielectric.
In a further embodiment, the thermosetting compositions of U.S. Pat. No. 5,233,568 and U.S. application Ser. No. 08/238,178, as described above, may further be used to provide thermally conductive coatings for electrical resistance heating elements. Accordingly, an electrical resistance heating element is described, that preferably comprises an electrically conductive, resistance heating member having a pair of free ends joined to a pair of terminal end portions, the resistance heating member being supported by and encapsulated within an integral layer of an electrically insulating, thermally conductive polymeric material, wherein the polymeric layer is formed from a polymerizable composition comprising a polybutadiene or polyisoprene resin. In another embodiment, the polymer coating is formed from 1) a polybutadiene or polyisoprene resin that is liquid at room temperature and that has a molecular weight less than 5,000 and a large number of pendent vinyl groups; and 2) a solid butadiene or isoprene containing polymer such as a thermoplastic elastomer.
In the method of manufacture of the polymer-coated heating element, one of the two above-described compositions is coated onto an electrical resistance heating element by overmolding using processes such as compression molding or injection molding.
The above-discussed and other features and advantages of the present invention will be appreciated and understood by those of ordinary skill in the art from the following detailed description and drawings.